The present invention relates to a reducing agent supply apparatus that supplies a liquid reducing agent in the exhaust passage of an internal combustion engine and makes control so as to suck the liquid reducing agent remaining in a reducing agent supply path back to a storage tank when an internal combustion engine stops and a method for controlling such a reducing agent supply apparatus.
Nitrogen oxide (NOX) is included in exhaust gas of an internal combustion engine installed in a vehicle or the like. As one exhaust purifying apparatus for purifying NOX, there is a known exhaust purifying apparatus that includes a selective reduction catalyst in the exhaust passage of the internal combustion engine and a reducing agent supply apparatus for injecting a liquid reducing agent such as an aqueous urea solution derived from ammonia upstream of the selective reduction catalyst. This exhaust purifying apparatus performs the reduction reaction of NOX in exhaust gas and ammonia generated from the liquid reducing agent in the selective reduction catalyst to efficiently decompose NOX to nitrogen, water, or the like.
As an aspect of a reducing agent supply apparatus used in such an exhaust purifying apparatus, there is a reducing agent supply apparatus that includes a pump and a reducing agent injection valve, pressure-feeds the liquid reducing agent in the storage tank using the pump, and supplies the liquid reducing agent into the exhaust pipe via the reducing agent injection valve fixed to an exhaust pipe.
When an aqueous urea solution is used as the liquid reducing agent, the aqueous urea solution preferably has a concentration that makes the freeze temperature lowest to prevent the freezing of the aqueous urea solution as much as possible. However, the freeze temperature of an aqueous urea solution is at least approximately −11 degrees and the aqueous urea solution may freeze in the time period in which the supplying of an aqueous urea solution is stopped in cold climate areas or the like. In addition, in the time period in which the supplying of an aqueous urea solution is stopped, the concentration increases because water in the aqueous urea solution evaporates and the melting point of the aqueous urea solution rises, possibly causing the freezing easily.
When the aqueous urea solution freezes, the unfreeze time at the next startup becomes long or the expanded volume may damage components of the reducing agent supply apparatus. Accordingly, when the internal combustion engine is stopped, control is generally performed so that the aqueous urea solution remaining in the reducing agent supply apparatus is sucked back to the storage tank. The suck back control of the aqueous urea solution is performed by reversely rotating the pump for pressure-feeding the aqueous urea solution or performing switching between passages of the aqueous urea solution to decompress the supply path for the aqueous urea solution. The suck back control of the aqueous urea solution is performed by driving the pump with the reducing agent injection valve opened for a predetermined period (for example, see JP-A-2010-007617).